Writes with an important fact about the parallel roads of Glen Roy. The watershed at Makoul corresponds with the lowermost of the Glen Roy lines. Over a stretch of 20 miles from east to west the lowermost of the Glen Roy lines is near parallel with the present sea level.

Transcription

Ellon Aberdeensh.

24 March 1862

My Dear Sir,

I have just lighted upon what I consider an important fact with reference to the
parallel roads of Glen Roy & wh. may probably interest
you.

In the letter I wrote you after my return from Lochaber I think I mentioned that owing
to the impossibility of tracing the lowermost line along the shores of Loch Laggan with
any degree of certainty I felt somewhat in doubt as to the fact of the outlet at Makoul
coinciding with that line— I afterwards wrote to
both Mr. Chambers & Mr. Milne Home on the subject
but failed to elicit anything certain on the subject.

I had however little doubt in my own mind that Loch Laggan had formerly discharged its
waters E,ward by that channel from the circumstance of the great old delta of the Gulban
which fills the S.W. corner of the lake & rises high above its present
level.

—But by the greatest good fortune I find that the ordnance survey had run a
line of spirit levelling up Glen Spean on to Dalwhinnie. and from the record of
the engineers it appears that the water shed at Makoul just coincides with the lowermost
of the Glen Roy lines— Thus, the height of said line found by the spirit
levelling of an engineer employed for Mr. R. Chambers is
847 feet above the sea it is not stated whether this refers
to high water or mean sea level, but most of these surveyors usually take high water
mark. and the summit level of the road at the water-shed of Makoul according to
the Ordnance surveyors is 850 feet above the
mean sea
level.

—Now if the sea level taken by Mr. Chambers' surveyor was high
water mark it wd. make the Glen Roy line up somewhat over
850 feet— something also would depend upon what part of the line the
surveyor took as the line of terrace is generally several feet in vertical height.

Anyhow the coincidence is too close to be accidental—

I am inclined to think however that the level of the outflowing water at Makoul had
been at one time fully 860 feet. judging from some measurements
wh. I made of the height of the old delta at the
S. W. corner of L. Laggan. & also from my remembrance
of the beds of waterworn pebbles &c at the Makoul outlet
wh. rise some feet I think above the summit level of the road.
—Whether this is owing to some inequality in the level of the land that has
occurred since the period of the Glen Roy roads—or whether it is owing to some
discrepancy in the levellings I cannot say—

But I think it is evident that over a stretch of 20 miles from E. to
W. this lowermost of the Glen Roy lines is wonderfully near if not altogether
parallel to the present sea level, and I think there can be now very little doubt that
it coincides with this outlet & has been determined by it.

If you think Sir C. Lyell wd. be interested in this
communication I should feel much obliged if you wd. show it him
as it wd. save me writing it over again.

The so-called `parallel roads' of Glen Roy are three terraces that run parallel to
one another along the sides of Glen Roy in Lochaber, Scotland. In the nineteenth century
several attempts were made to account for their formation; in a paper published in 1839
(`Parallel roads of Glen Roy'), CD argued that the roads were the remains of beaches
formed by the sea as the landmass of Scotland rose in graduated steps. By contrast,
Jamieson believed that during a great `Ice Age' ice-flows trapped a series of lakes in
the glen and that the roads represented the shorelines of three of these former lakes.
M. J. S. Rudwick's account of what CD regarded as his
`gigantic blunder' in relation to Glen Roy details the major explanations proposed
during the first half of the nineteenth century to provide this celebrated phenomenon
with a natural history (Rudwick 1974).

+

f2 3483f.f2

See Correspondence vol. 9, letter from
T. F. Jamieson, 3 September 1861. Loch Laggan is in
Glen Spean, a glen adjacent to Glen Roy. The lowermost of the Glen Roy roads continues
out of Glen Roy and around Glen Spean.